Virtual locating of a fixed subscriber unit to reduce re-acquisition time

ABSTRACT

A novel system and method reduces the time required by a base station to re-acquire a fixed subscriber unit in a CDMA communication system by virtually locating of the subscriber units. A base station acquires subscriber units by searching only those code phases concomitant with the largest propagation delay possible in the cell, as if all subscriber units were located at the periphery of the cell. A subscriber unit which has never been acquired by the base station varies the delay between the PN code phase of its received and transmitted signals over the range of possible delays in a cell and slowly ramps-up its transmission power until it is acquired by the base station. Upon initial acquisition by the base station, the subscriber unit ceases ramping-up its transmission power, ceases varying the delay and internally stores the final value of the delay in memory. For subsequent re-acquisition, the subscriber unit adds the delay value between the PN code phase of its received and transmitted signals, making the subscriber virtually appear to be at the periphery of the cell. This permits a quick ramp-up of transmission power by the subscriber unit and reduced acquisition time by the base station.

CROSS REFERENCE TO RELATED APPLICATION

This application is being filed concurrently with an applicationentitled Code Division Multiple Access (CDMA) System and Method which isherein incorporated by reference as if fully set forth.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to code division multiple access(CDMA) communication systems. More particularly, the present inventionrelates to a CDMA communication system which utilizes virtual locatingof a fixed subscriber unit to reduce the time for a base station todetect an access signal from a subscriber unit and establish acommunication channel between the base station and the subscriber unit.

2. Description of Related Art

Most widely used conventional telecommunication systems requiretransmissions to be confined to a separate frequency or time slot.Systems using frequency division multiple access (FDMA) assign each usera specific portion of the frequency spectrum for communication. Systemsusing time division multiple access (TDMA) assign each user a repeatingtime slot to transmit the desired information. These conventionaltechniques require strict definition of time slots, channels andguardbands between channels in order to prevent communicating nodes frominterfering with one another.

Channelization and guardband requirements have resulted in a tremendousinefficiency in the use of the RF spectrum. As the number of commercialapplications of wireless technology increases, the need forcommunication systems which utilize the RF spectrum more efficiently hasbecome paramount.

CDMA communication systems have had a long history of use in militaryapplications. CDMA permits communications which are difficult to detectby enemies and offer robust communications during attempts by enemies tojam communications. In CDMA communications, each signal or communicationchannel is distinguished from all others in a particular frequency bandby a unique pseudo noise (PN) code imprinted upon data transmitted bythe transmitter. A receiver which is privy to the unique code uses thecode to resolve the desired data signal channel from among many thesimultaneous data signals and channels in the frequency band.

The features that have enabled CDMA communication systems to succeed inmilitary applications also make CDMA communication systems well adaptedfor efficiently utilizing the RF spectrum. Since each subscriber unit ina CDMA communication system transmits and receives resolvablecommunication signals over the same frequency band, there are lessstringent channelization and guardband requirements. Accordingly, thecapacity of the system (the number of users able to communicatesimultaneously) is significantly increased.

Although use of the same portion of the RF spectrum by a plurality ofsubscriber units increases system efficiency, each subscriber unitreceives communication signals that do not have its unique code asinterference. The more power that is utilized by a single subscriberunit to communicate with the base station, the more interference ispresented to other subscriber units. The power from one subscriber unitmay even terminate other communications if it becomes too high.Accordingly, the control of the transmission power of all subscriberunits is important to maintain high quality communications throughoutthe system.

A typical CDMA communication system is shown in FIG. 1. The systemcomprises a cell base station (B), and a plurality of fixed subscriberunits S1-S7 located at various distances from the base station. The basestation constantly transmits a forward pilot signal. The subscriberunits maintain epoch alignment between the forward pilot signal andtheir internal PN code generator such that all signals transmitted fromthe subscriber unit are at the same PN code phase at which the forwardpilot is received. The base station receives signals from subscriberunits with a code phase difference between its forward pilot signal andthe received signal corresponding to the two-way signal propagationdelay between the base station and the subscriber.

For the base station to detect a signal, it must align the phase of itsreceive PN code generator to the phase of the received signal, thus"acquiring" the signal. The base station can receive an access signalwith any code phase difference within the range of the cell. Therefore,the base station must test all code phases associated with the range ofpossible propagation delays of the cell to acquire the access signal.

Once a communication channel is established between the base station andthe subscriber unit, the transmission power of the subscriber unit iscontrolled by a closed loop automatic power control (APC) algorithmwhich prevents the power from each subscriber unit from excessivelyinterfering with other subscriber units. During channel establishment,before the closed loop power control begins, the subscriber unit'stransmission power is kept to a minimum by ramping-up from a low leveland establishing the channel without the subscriber unit significantlyovershooting (on the order of less than 3 dB) the minimum powernecessary to operate the channel.

To establish a channel, each subscriber unit transmits a PN coded accesssignal for detection by the base station. The base station acquires theaccess signal and transmits a confirmation signal to each subscriberunit. The time required for the base station to acquire the accesssignal contributes directly to the time elapsed between a subscriberunit going "off-hook", establishing a communication channel, connectingto the public switched telephone network (PSTN) and receiving a dialtone. It is desirable to receive a dial tone within 150 msec ofdetection of "off-hook".

The time distribution of acquisition opportunities is shown in FIG. 2for a typical subscriber unit located 20 km from a base station in a 30km cell. For a base station which tests 8 code phases simultaneously ata PN rate of 12.48 MHz and a symbol rate of 64,000 symbols per second,and takes an average of 7.5 symbol periods to accept or reject aparticular group of code phases, the average time to test all code phasedelays within the cell is approximately 37 msec, and any one subscriberunit can only be detected during an approximately 100 μsec window duringthat period. Assuming that the selection of initial subscriber unittransmission power level is 15-20 dB below the proper level and a slowramp-up rate of between 0.05 and 0.1 dB/msec, it could take 4-5 such 37msec time periods, (or an average of approximately 200 msec,) for thebase station to acquire a subscriber unit. This introduces anunacceptable delay in the channel establishment process which should beless than 150 msec.

Accordingly, there is a need to reduce the amount of time required for abase station to acquire a subscriber unit.

SUMMARY OF THE INVENTION

The present invention comprises a method of reducing the re-acquisitiontime of a fixed subscriber unit by a base station in a CDMAcommunication system by utilizing virtual locating of the subscriberunit. A base station acquires subscriber units by searching only thosecode phases concomitant with the largest propagation delay possible inthe cell, as if all subscriber units were located at the periphery ofthe cell. A subscriber unit which has never been acquired by the basestation varies the delay between the PN code phase of its received andtransmitted signals over the range of possible delays in a cell andslowly ramps-up its transmission power until it is acquired by the basestation. Upon initial acquisition by the base station the subscriberunit ceases ramping-up its power and varying the delay and internallystores the final value of the delay in memory. For subsequentre-acquisition, the subscriber unit adds the delay value between the PNcode phase of its received and transmitted signals, making thesubscriber virtually appear to be at the periphery of the cell. Thispermits a quick ramp-up of transmission power by the subscriber unit andreduced acquisition time by the base station.

Accordingly, it is an object of the present invention to provide animproved method and system for decreasing the reacquisition time of afixed subscriber unit by a base station in a CDMA communication system.

Other objects and advantages of the present invention will becomeapparent after reading the description of a presently preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art CDMA communication system;

FIG. 2 is a graph of the distribution of acquisition opportunities ofthe system of FIG. 1;

FIG. 3 is a schematic overview of a CDMA communication system inaccordance with the present invention;

FIG. 4 is a diagram showing the propagation of signals between a basestation and a plurality of subscriber units;

FIG. 5 is a flow diagram of the preferred embodiment of the initialestablishment of a communication channel between a base station and asubscriber unit using slow initial acquisition;

FIG. 6 is a flow diagram of the preferred embodiment of thereestablishment of a communication channel between a base station and asubscriber unit using fast re-acquisition;

FIG. 7A is a diagram of the communications between a base station and aplurality of subscriber units;

FIG. 7B is a diagram of the base station and a subscriber unit which hasbeen virtually located;

FIG. 8 is a schematic overview of a plurality of subscriber units whichhave been virtually located;

FIG. 9 is a subscriber unit made in accordance with the teachings of thepresent invention;

FIG. 10 is a flow diagram of an alternative embodiment of the initialestablishment of a communication channel between a base station and asubscriber unit using slow initial acquisition;

FIG. 11 is a flow diagram of an alternative embodiment of thereestablishment of a communication channel between a base station and asubscriber unit using fast re-acquisition; and

FIG. 12 is a flow diagram of a second alternative embodiment of theinitial establishment of a communication channel between a base stationand a subscriber unit using slow initial acquisition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment will be described with reference to the drawingfigures where identical numerals represent similar elements throughout.

A communication network 10 embodying the present invention is shown inFIG. 3. The communication network 10 generally comprises one or morebase stations 14, each of which is in wireless communication with aplurality of fixed subscriber units 16. Each subscriber unit 16communicates with either the closest base station 14 or the base station14 which provides the strongest communication signal. The base stations14 also communicate with a base station controller 20, which coordinatescommunications among base stations 14 and between base stations 14. Thecommunication network may also be connected to a public switchedtelephone network (PSTN) 22, whereupon the base station controller 20also coordinates communication between the base stations 14 and the PSTN22. Preferably, each base station 14 communicates with the base stationcontroller 20 over a wireless link, although a land line may also beprovided. A land line is particularly applicable when a base station 14is in close proximity to the base station controller 20.

The base station controller 20 performs several functions. Primarily,the base station controller 20 provides all of the overhead,administrative and maintenance (OA&M) signaling associated withestablishing and maintaining all of the wireless communications betweenthe subscriber units 16, the base stations 14, and the base stationcontroller 20. The base station controller 20 also provides an interfacebetween the wireless communication system 10 and the PSTN 22. Thisinterface includes multiplexing and demultiplexing of the communicationsignals that enter and leave the system 10 via the base stationcontroller 20. Although the wireless communication system 10 is shownemploying antennas to transmit RF signals, one skilled in the art shouldrecognize that communications may be accomplished via microwave orsatellite uplinks. Additionally, the functions of a base station 14 maybe combined with the base station controller 20 to form a master basestation. The location of where these base station controller functionsare performed is not central to the present invention.

Referring to FIG. 4, the propagation of certain signals in theestablishment of a communication channel 18 between a base station 14and a plurality of subscriber units 16 is shown. The forward pilotsignal 20 is transmitted from the base station 14 at time t0, and isreceived by a subscriber unit 16 after a propagation delay Δt. To beacquired by the base station 14 the subscriber unit 16 transmits anaccess signal 22 which is received by the base station 14 after afurther propagation delay of Δt. Accordingly, the round trip propagationdelay is 2Δt. The access signal 22 is transmitted epoch aligned to theforward pilot signal 20, which means that the code phase of the accesssignal 22 when transmitted is identical to the code phase of thereceived forward pilot signal 20.

The round trip propagation delay depends upon the location of asubscriber unit 16 with respect to the base station 14. Communicationsignals transmitted between a subscriber unit 16 located closer to thebase station 14 will experience a shorter propagation delay than asubscriber unit 16 located further from the base station 14. Since thebase station 14 must be able to acquire subscriber units 16 located atany position within the cell 30, the base station 14 must search allcode phases of the access signal corresponding to the entire range ofpropagation delays of the cell 30.

It should be apparent to those of skill in the art that theestablishment of a communication channel between a base station 14 and asubscriber unit 16 is a complex procedure involving many tasks performedby the base station 14 and the subscriber unit 16 which are outside thescope of the present invention. The present invention is directed todecreasing the reacquisition time of a fixed subscriber unit 16 by abase station 14 during the re-establishment of a communication channel.

Referring to FIG. 5, the tasks associated with initial acquisition of asubscriber unit 16 by a base station 14 in accordance with the preferredembodiment of the present invention are shown. When a subscriber unit 16desires the establishment of a channel 18 with a base station 14 withwhich it has never established a channel, the subscriber unit 16 has noknowledge of the two-way propagation delay. Accordingly, the subscriberunit 16 enters the initial acquisition channel establishment process.

The subscriber unit 16 selects a low initial power level and zero codephase delay, (epoch aligning the code phase of the transmitted accesssignal 22 to the code phase of the received forward pilot signal 20),and commences transmitting the access signal 22 while slowly (0.05-0.1dB/msec) ramping-up transmission power (step 100). While the subscriberunit 16 is awaiting receipt of the confirmation signal from the basestation 14, it varies the code phase delay in predetermined steps fromzero to the delay corresponding to the periphery of the cell 30, (themaximum code phase delay), allowing sufficient time between steps forthe base station 14 to detect the access signal 22 (step 102). If thesubscriber unit 16 reaches the code phase delay corresponding to theperiphery of the cell 30, it repeats the process of varying the codephase delay while continuing the slow power ramp-up (step 102).

In order to acquire subscriber units 16 desiring access, the basestation 14 continuously transmits a forward pilot signal 20 and attemptsto detect the access signals 22 from subscriber units 16 (step 104).Rather than test for access signals 22 at all code phase delays withinthe cell 30 as with current systems, the base station 14 need only testcode phase delays centered about the periphery of the cell 30.

The base station 14 detects the access signal 22 (step 106) when thesubscriber unit 16 begins transmitting with sufficient power at the codephase delay which makes the subscriber unit 16 appear to be at theperiphery of the cell 30, thereby "virtually" locating the subscriberunit 16 at the periphery of the cell 30. The base station 14 thentransmits a signal to the subscriber unit 16 which confirms that theaccess signal 22 has been received (step 108) and continues with thechannel establishment process (step 110).

Once the subscriber unit 16 receives the confirmation signal (step 112),it ceases the ramp-up of transmission power, ceases varying the codephase delay (step 114) and records the value of the code phase delay forsubsequent re-acquisitions (step 116). The subscriber unit 16 thencontinues the channel establishment process including closed-loop powertransmission control (step 118).

On subsequent re-acquisitions when a subscriber unit 16 desires theestablishment of a channel 18 with a base station 14, the subscriberunit 16 enters the re-acquisition channel establishment process shown inFIG. 6. The subscriber unit 16 selects a low initial power level and thecode phase delay recorded during the initial acquisition process, (shownin FIG. 5), and commences continuously transmitting the access signal 22while quickly (1 dB/msec) ramping-up transmission power (step 200).While the subscriber unit 16 is awaiting receipt of the confirmationsignal from the base station 14, it slightly varies the code phase delayof the access signal 22 about the recorded code phase delay, allowingsufficient time for the base station 14 to detect the access signal 22before changing the delay (step 202). The base station 14 as in FIG. 5,transmits a forward pilot signal 20 and tests only the code phase delaysat the periphery of the cell 30 in attempting to acquire the subscriberunits 16 within its operating range (step 204). The base station 14detects the access signal 22 when the subscriber unit 16 transmits withsufficient power at the code phase delay which makes the subscriber unit16 appear to be at the periphery of the cell 30 (step 206). The basestation 14 transmits a signal to the subscriber unit 16 which confirmsthat the access signal 22 has been received (step 208) and continueswith the channel establishment process (step 210).

When the subscriber unit 16 receives the confirmation signal (step 212)it ceases power ramp-up, ceases varying the code phase delay (step 214)and records the present value of the code phase delay for subsequentre-acquisitions (step 216). This code phase delay may be slightlydifferent from the code phase delay initially used when starting there-acquisitions process (step 202). The subscriber unit 16 thencontinues the channel establishment process at the present power level(step 218). If a subscriber unit 16 has not received a confirmationsignal from the base station 14 after a predetermined time, thesubscriber unit 16 reverts to the initial acquisition process describedin FIG. 5.

The effect of introducing a code phase delay in the Tx 20 and Rx 22communications between the base station 14 and a subscriber unit 16 willbe explained with reference to FIGS. 7A and 7B. Referring to FIG. 7A, abase station 160 communicates with two subscriber units 162, 164. Thefirst subscriber unit 162 is located 30 km from the base station 160 atthe maximum operating range. The second subscriber unit 164 is located15 km from the base station 160. The propagation delay of Tx and Rxcommunications between the first subscriber unit 162 and the basestation 160 will be twice that of communications between the secondsubscriber unit 164 and the base station 160.

Referring to FIG. 7B, after an added delay value 166 is introduced intothe Tx PN generator of the second subscriber unit 164 the propagationdelay of communications between the first subscriber unit 162 and thebase station 160 will be the same as the propagation delay ofcommunications between the second subscriber unit 164 and the basestation 160. Viewed from the base station 160, it appears as though thesecond subscriber unit 164 is located at the virtual range 164'.

Referring to FIG. 8, it can be seen that when a plurality of subscriberunits S1-S7 are virtually relocated S1'-S7' to the virtual range 175,the base station must only test the code phase delays centered about thevirtual range 175.

Utilizing the present invention, a subscriber unit 16 which has achieveda sufficient power level will be acquired by the base station 14 inapproximately 2 msec. Due to the shorter acquisition time, thesubscriber unit 16 can ramp-up at a much faster rate, (on the order of 1dB/msec), without significantly overshooting the desired power level.Assuming the same 20 dB power back-off, it would take the subscriberunit 16 approximately 20 msec to reach the sufficient power level fordetection by the base station 14. Accordingly, the entire duration ofthe re-acquisition process of the present invention is approximately 22msec, which is an order of magnitude reduction from prior artreacquisition methods.

A subscriber unit 200 made in accordance with the present invention isshown in FIG. 9. The subscriber unit 200 includes a receiver section 202and a transmitter section 204. An antenna 206 receives a signal from thebase station 14, which is filtered by a band-pass filter 208 having abandwidth equal to twice the chip rate and a center frequency equal tothe center frequency of the spread spectrum system's bandwidth. Theoutput of the filter 208 is down-converted by a mixer 210 to a basebandsignal using a constant frequency (Fc) local oscillator. The output ofthe mixer 210 is then spread spectrum decoded by applying a PN sequenceto a mixer 212 within the PN Rx generator 214. The output of the mixer212 is applied to a low pass filter 216 having a cutoff frequency at thedata rate (Fb) of the PCM data sequence. The output of the filter 216 isinput to a codec 218 which interfaces with the communicating entity 220.

A baseband signal from the communicating entity 220 is pulse codemodulated by the codec 218. Preferably, a 32 kilobit per second adaptivepulse code modulation (ADPCM) is used. The PCM signal is applied to amixer 222 within a PN Tx generator 224. The mixer 222 multiplies the PCMdata signal with the PN sequence. The output of the mixer 222 is appliedto low-pass filter 226 whose cutoff frequency is equal to the systemchip rate. The output of the filter 226 is then applied to a mixer 228and suitably up-converted, as determined by the carrier frequency Fcapplied to the other terminal. The up-converted signal is then passedthrough a band-pass filter 230 and to a broadband RF amplifier 232 whichdrives an antenna 234.

The microprocessor 236 controls the acquisition process as well as theRx and Tx PN generators 214, 224. The microprocessor 236 controls thecode phase delay added to the Rx and Tx PN generators 214, 224 toacquire the forward pilot signal 20, and for the subscriber unit 200 tobe acquired by the base station 14, and records the code phasedifference between these PN generators. For re-acquisition themicroprocessor 236 adds the recorded delay to the Tx PN generator 224.

The base station 14 uses a configuration similar to the subscriber unit16 to detect PN coded signals from the subscriber unit 200. Themicroprocessor (not shown) in the base station 14 controls the Rx PNgenerator in a similar manner to make the code phase difference betweenRx PN generator and the Tx PN generator equivalent to the two-waypropagation delay of the subscriber unit's 16 virtual location. Once thebase station 14 acquires the access signal 22 from the subscriber unit16, all other signals from the subscriber unit 16 to the base station 14(traffic, pilot, etc.) use the same code phase delay determined duringthe acquisition process.

It should be noted that although the invention has been described hereinas the virtual locating of subscriber units 16 at the periphery of thecell 30 the virtual location can be at any fixed distance from the basestation 14.

Referring to FIG. 10, the tasks associated with initial acquisition of a"never-acquired" subscriber unit 16 by a base station 14 in accordancewith an alternative embodiment of the present invention are shown. Thesubscriber unit 16 continuously transmits an epoch aligned access signal22 to the base station 14 (step 300) when the establishment of a channel18 is desired. While the subscriber unit 16 is awaiting the receipt of aconfirmation signal from the base station 14, it continuously increasesthe transmission power as it continues transmission of the access signal22 (step 302).

To detect subscriber units which have never been acquired, the basestation 14 transmits a forward pilot signal 20 and sweeps the cell bysearching all code phases corresponding to the entire range ofpropagation delays of the cell (step 304) and detects the epoch alignedaccess signal 22 sent from the subscriber unit 16 after the transmissionhas achieved sufficient power for detection (step 306). The base station14 transmits a signal to the subscriber unit 16 (step 308) whichconfirms that the access signal 22 has been received. The subscriberunit 16 receives the confirmation signal (step 310) and ceases theincrease in transmission power (step 312).

The base station 14 determines the desired code phase delay of thesubscriber unit 16 by noting the difference between the Tx and Rx PNgenerators 224, 214 after acquiring the subscriber unit 16. The desiredcode phase delay value is sent to the subscriber unit 16 (step 316) asan OA&M message, which receives and stores the value (step 318) for useduring re-acquisition, and continues with the channel establishmentprocess (steps 322 and 324).

Referring to FIG. 11, an alternative method of fast reacquisition inaccordance with the present invention is shown. When a communicationchannel must be reestablished between the subscriber unit 16 and thebase station 14, the subscriber unit 16 transmits the access signal 22with the desired code phase delay as in the preferred embodiment.

With all of the previously acquired subscriber units 16 at the samevirtual range, the base station 14 need only search the code phasedelays centered about the periphery of the cell to acquire the accesssignals 22 of such subscriber units 16 (step 330). Thus, a subscriberunit 16 may ramp-up power rapidly to exploit the more frequentacquisition opportunities. The subscriber unit 16 implements the delaythe same way as in the preferred embodiment. The base station 14subsequently detects the subscriber unit 16 at the periphery of the cell(step 336), sends a confirmation signal to the subscriber unit (step337) and recalculates the desired code phase delay value, if necessary.Recalculation (step 338) compensates for propagation path changes,oscillator drift and other communication variables. The base station 14sends the updated desired code phase delay value to the subscriber unit16 (step 340) which receives and stores the updated value (step 342).The subscriber unit 16 and the base station 14 then continue the channelestablishment process communications (steps 344 and 346).

Note that the alternative embodiment requires the base station to searchboth the code phase delays centered on the periphery of the cell tore-acquire previously acquired subscriber units and the code phasedelays for the entire cell to acquired subscriber units which have neverbeen acquired.

Referring to FIG. 12, the tasks associated with initial acquisition of anever-acquired subscriber unit 16 by a base station 14 in accordancewith a second alternative embodiment of the present invention are shown.In the embodiment shown in FIG. 10, when a never-acquired subscriberunit 16 is acquired the access signal 20 remains epoch aligned to theforward pilot signal 20. In this embodiment, the base station 14 andsubscriber unit 16 change the code phase alignment of the access signal22 from epoch aligned to delayed, (by the code phase delay), to make thesubscriber unit 16 appear at the periphery of the cell. This change isperformed at a designated time.

Steps 400 through 418 are the same as the corresponding steps 300through 318 shown in FIG. 10. However, after the base station 14 sendsthe desired delay value to the subscriber unit 16 (step 416) the basestation 14 sends a message to the subscriber unit 16 to switch to thedesired delay value at a time referenced to a sub-epoch of the forwardpilot signal 20 (step 420). The subscriber unit 16 receives this message(step 422), and both units 14, 16 wait until the switchover time isreached (steps 424, 430). At that time, the base station 14 adds thedesired delay value to its Rx PN operator (step 432) and the subscriberunit 16 adds the same desired delay value to its Tx PN generator (step426). The subscriber unit 16 and the base station 14 then continue thechannel establishment process communication (step 428, 434).

Although the invention has been described in part by making detailedreference to the preferred and alternative embodiments, such detail isintended to be instructive rather than restrictive. It will beappreciated by those skilled in the art that many variations may be madein the structure and mode of operation without departing from the spiritand scope of the invention as disclosed in the teachings herein.

What is claimed is:
 1. A method for reducing the reacquisition time of asubscriber unit by a base station in a network for communicating betweena base station and at least one subscriber unit; the methodcomprising:transmitting an access signal from a subscriber unit at apredetermined power transmission level; increasing said predeterminedpower transmission level until a confirmation signal is received fromsaid base station; detecting said access signal at said base stationwhen a sufficient power level has been achieved; transmitting aconfirmation signal from said base station when said access signal hasbeen detected; receiving said confirmation signal at said subscriberunit; ceasing the increase in transmission power from said subscriberunit when said confirmation signal is received; calculating a delayvalue at said subscriber unit between said access signal and saidconfirmation signal; and storing said delay for subsequent subscriberunit reacquisitions.
 2. The method of claim 1 wherein said subscriberunit selectively increases said power level.
 3. The method of claim 2wherein the step of detecting at said base station further includesanalyzing said access signal at a plurality of code phases.
 4. Themethod of claim 3 further including generating a confirmation signal atsaid base station for transmission to said subscriber unit when acommunication from the subscriber unit has been detected.
 5. The methodof claim 4 further including detecting, at said subscriber unit, aconfirmation signal from said base station.
 6. The method of claim 5further including determining, at said base station, the durationbetween the transmission of a communication sent to said subscriber unitand the receipt of responding communication from said subscriber.
 7. Themethod of claim 6 wherein said step of determining further includescalculating the difference between said determined duration and adesired duration.
 8. The method of claim 7 further includingtransmitting, from said base station, a difference signal to saidsubscriber unit based upon said calculated difference.
 9. The method ofclaim 8 further including receiving, at said subscriber unit, saiddifference signal and delaying signals transmitted from said subscriberunit by said difference.